Field of the Invention
In the techniques of electrophotographic photosensitive members, the maximum value of ratio of the change of the a surface potential (.DELTA.V) to the change of exposure E (hereinafter represented by .DELTA. log E) is very important. This maximum value is called .gamma. value. The above-mentioned relation is represented by the following formula. EQU .vertline..DELTA.V/.DELTA. log E.vertline..sub.max =.gamma.
The .gamma. value has a great effect on the reproducibility of original information during latent image formation in electrophotographic processes. That is, where .gamma. value of a photosensitive member is very high, the resulting latent images are very unstable since the surface potential is very sensitive to even a minor change of exposure and, in particular, latent images corresponding to a light of intermediate intensity are so unstable that the developed images can not contain any half tone (grey) and are of low gradation.
On the contrary, where .gamma. value is too small, fog is formed and good images can not be obtained. In general, .gamma. value is controlled by doping with impurities such as Cu, Cl, In and the like and adjusting the particle size of photoconductive particles in a photoconductive layer. However, such procedures do not always give a satisfactory result.
In addition, in the case of an electrophotographic photosensitive member having a photoconductive layer containing photoconductive particles, the .gamma. value usually tends to increase during repeated use and the rate of change of .gamma.-value is a main factor to estimate durability of a photosensitive member.
At present, production of photosensitive members of a low change rate of .gamma. value is a big problem to be solved.